If its the color of the name, then it is taken! Just ask if you want the role! If role is taken, then you can be one like aqua, which isn't exactly blue. Or you could be Ruby or something.
Red Stone - Yuzuki Nakahara/Yuzuki Nakahara
Orange Stone
Yellow Stone - Kotori Harada/Kotori
Green Stone
Blue Stone
Purple Stone
Pink Stone - Uta Rabula/Admin
Gray Stone